Contractile cardiac patches offer the promise of improving mechanical function and clinical outcomes for diseased or damaged myocardium following a myocardial infarct (MI). However, currently available acellular scaffolds fail to couple with the host tissue and only provide passive structural support. As such, there remains a significant need to engineer an active cardiac patch that provides structural stability to damaged ventricular myocardium, generates cell-mediated contractility, and couples to the host myocardium, providing a substrate conducive to functional tissue regeneration. Recently, we developed novel methods for 3D printing fibrin microthreads. These fibers support human-induced pluripotent stem cell -derived cardiomyocyte (iPS-CM) attachment, providing up to 10% contraction. In addition, signal propagation along the lengths of the fibers has demonstrated a conduction velocity of 2.1 cm/s. These cell-seeded fibers have structural, mechanical and biochemical cues that can be precisely designed into hierarchically organized structures. From these observations, we hypothesize that cell-mediated contractile fibrin microthreads can be engineered into sheets of composite, contractile myocardial tissue that can be electrically coupled to provide signal propagation rates similar to native myocardial tissue. To test this hypothesis, we propose the following specific aims: In the first aim, we will maximize contractility of iPS-CM seeded fibrin microthreads with a physiologically relevant conduction velocity. We will systematically modulate fibrin microthread diameters and crosslinking strategies and we will determine the structural morphology that maximizes iPS-CM-mediated alignment, contractility and conduction velocities on the surfaces of the microthreads. In the second aim, we will maximize contractile function of iPS-CMs seeded microthread-based composite layers. We will develop anisotropically aligned composite layers and we will modulate the packing density of iPS- CM seeded threads in the composite to determine the scaffold morphology that maximizes layer contractility and signal propagation rates In our final aim, we will laminate iPS-CMs seeded microthread-based layers to produce electrically coupled contractile cardiac patches with dimensions of 1.0 cm x 1.0 cm x 0.3 mm thick. We will couple three individual composite layers with a cell-seeded microthread, creating scaffolds and we will determine the relative orientations of the layers that maximize the synchronous contractile properties. The results of these proposed studies will enable us to systematically determine design parameters to maximize the iPS-CM mediated synchronous contractility of a physiologically relevant cardiac patch, composed of laminated sheets of aligned, microthread-based engineered myocardial tissue.